Sir, We are grateful for the opportunity to respond to the correspondence from Dr He, and we thank the author for his interest in our recent article, which demonstrated that platelet-derived growth factor (PDGF) is secreted by bone marrow-derived mesenchymal stem cells (MSCs) and confers neuroprotection to retinal ganglion cells in organotypic retinal explant culture and in an in vivo model of ocular hypertensive glaucoma (Johnson et al., 2013). Based on our previously reported observation that intravitreal MSC transplantation confers optic nerve neuroprotection (Johnson et al., 2010), we recently analysed the human MSC secretome and compared it to that of human fibroblasts using xMAP antibody-based arrays. As Dr He noted, we demonstrated that PDGF-AA secretion from MSCs was >500-fold greater than from fibroblasts, …